Wireless Local Area Networks (WLANs) have recently gained in popularity and importance. These networks are a special case of standard computer Local Area Networks (LANs), wherein the wires or optical fibers interconnecting computers have been partially or completely replaced by radio frequency (RF) data links operating at very high frequencies. WLANs may also be viewed as a special case of commonly encountered cellular telephone networks, where the relatively large distances (tens of miles) covered by cellular telephones have been significantly reduced (to hundreds of feet, in an indoor environment within buildings) in exchange for much higher data transmission rates. WLANs offer the possibility of interconnecting intra-building information technology devices such as computers, Personal Digital Assistants, printers, etc. at relatively high speeds without wires, and hence yield significant reductions in installation cost together with significant improvements in user convenience.
The design and installation of RF data links must factor in the propagation characteristics of the region covered by the data links. RF propagation characteristics of interest in indoor digital wireless communication systems include the following:                (a) Attenuation (loss) as a function of distance, including shadowing caused by reflecting or absorbing bodies in the environment; and        (b) Multipath (echoes) caused by reflection or diffraction from one or more scatterers in the environment, which can result in inter-symbol interference or variations in signal strength (fading).        WLANs are significantly affected by the propagation characteristics of the indoor environment in which they are used. This results from:        (a) Increased data rates. WLANs typically operate at data rates of 1 Mb/s or more for a given channel; modern WLAN technologies transfer data at speeds of up to 54 Mb/s. This means that the symbol periods used must be quite short, and consequently they are affected by inter-symbol interference resulting from multipath. In addition, the higher data rates require wider channels, which are more susceptible to effects such as frequency fading.        (b) Reduced transmit power limits. WLAN equipment is constrained to use unlicensed frequency bands with strict limits on radiated power, and as a result cannot overcome attenuation problems with increased transmit power.        (c) Strict cost and size constraints. As WLAN systems are intended to support mobile and portable applications in a home or business environment, it is not possible to deal with attenuation and multipath problems by utilizing large antenna arrays or complex networks of repeaters.        
The increased usage and reliance upon WLANs has in turn required a much greater emphasis to be placed on measuring the propagation characteristics of RF energy in an indoor environment. Heretofore, most RF propagation studies and analyses have focused on propagation characteristics in an outdoor (urban or regional) environment, in response to the needs of broadcasting, cellular telephony, and other fixed and mobile wireless systems. The indoor environment in which WLANs are placed, however, exhibits different propagation characteristics and requires measurements to be made in different ways than the outdoor environment.
The indoor RF propagation environment poses a number of challenges. Firstly, the environment is quite complex, containing a large number of scatterers as well as a high density of absorbing elements with diverse physical characteristics. Secondly, the environment typically changes frequently, as objects are moved about. Finally, the relatively large density of transmitters and receivers present in a WLAN environment results in a large number of interactions.
Several approaches have been implemented to date to enable these issues to be dealt with when implementing indoor data networks. These are:                (a) Computer modeling of the propagation characteristics of the indoor space based on an exact representation of the dimensions and physical characteristics of the contents. This entails locating all of the objects present in a floor or building (walls, ceilings, furniture, etc.), creating a computer model of the entire space, and then using the model to predict the propagation characteristics of interest to WLAN designers. While this yields accurate results, it is complex, time-consuming and expensive.        (b) Extrapolation based on previous propagation studies. A large number of studies have been made of the propagation characteristics of various kinds of buildings, as well as of the physical characteristics of various types of building materials. It is possible to combine the results of these studies to produce a composite model or a set of composite models that can be applied to the building of interest. However, this approach can produce very inaccurate and unpredictable results because the geometry and contents of buildings vary widely, and RF propagation is greatly affected by small differences in layout and composition.        (c) Empirical deduction based on signal strengths. This consists of setting up an RF signal source at some location in a building and then measuring signal strengths at various points within the floor or building, and deducing the propagation characteristics based on the various measurements. However, this method is labor intensive, error prone, and frequently unrepeatable.        (d) Direct measurement of the propagation characteristics. An apparatus known as a channel sounder can be employed to directly measure the propagation characteristics of an RF channel. Channel sounders have been commonly employed in measuring the characteristics of point-to-point wireless links, but have also been utilized in the propagation studies previously referred to in order to measure the characteristics of buildings and materials. However, no means has been disclosed in the prior art of using this approach to perform propagation measurements of building spaces in two (or three) dimensions.        
Accordingly, it is an object of the invention to provide an improved RF propagation measurement system for indoor environments. It is a further object of the invention to provide a propagation measurement system that enables measurements to be made over two-dimensional areas or regions. It is yet a further object of the invention to provide a propagation measurement system that allows the time-variant RF propagation characteristics of an indoor environment to be measured.